The measurement of the specific electrical conductivity is used for controlling process engineering processes. In food technology, for example, product streams in pipes are differentiated from cleaning solutions or rinsing water by means of the measurement of the specific electrical conductivity. Depending upon certain media, process engineering processes are also influenced.
In general, conductivity sensors that work according to an inductive or a conductive measurement principle are often used in the process automation to measure the electrical conductivity of a medium. A conductive conductivity sensor comprises at least two electrodes that are immersed in a medium in order to take measurements. In order to determine the electrical conductivity of the medium, the resistance or conductance of the electrode measuring path in the medium is determined. If the cell constant is known, the conductivity of the medium can then be determined. In order to measure the conductivity of a measuring fluid by means of a conductive conductivity sensor, it is absolutely necessary that at least two electrodes come into contact with the measuring fluid.
With the inductive principle of determining the conductivity of process media, sensors are used that comprise both a transmitter coil and a receiver coil arranged at a distance from the transmitter coil. By means of the transmitter coil, an alternating electromagnetic field is produced, which affects charged particles, e.g., ions, in the liquid medium and generates a corresponding electric current in the medium. As a result of this electric current, an electromagnetic field is generated at the receiver coil, inducing a received signal (induction voltage) in the receiver coil according to Faraday's law of induction. This received signal can be analyzed and used to determine the electrical conductivity of the liquid medium.
Inductive conductivity sensors are typically designed as follows. The transmitter coil and the receiver coil are generally built as toroidal coils and comprise a continuous opening through which the medium can flow. The coils are arranged in a housing which is immersed in the medium to be measured. The medium thus flows around both coils. The excitation of the transmitter coil creates in the medium a closed current path that passes through both the transmitter coil and the receiver coil. By analyzing the current and voltage signals of the receiver coil in response to the signal from the transmitter coil, the conductivity of the measuring fluid can be determined. The principle in itself is established in industrial process measurement technology and has been documented in a large number of texts in the patent literature.
The coils consist of at least one winding of a conductor made of a wire that is wound on a coil carrier and provided with a magnetic core. The winding arrangement and winding form, the diameter of the wire, the winding material, and the core material define the value of the respective inductance and additional (quality) characteristics of the coil.
High-quality coils and cores are often used for conductivity sensors. These coils have a low temperature dependency because the relative permeability of the coils or the cores exhibits a low temperature dependency. Even in cores of very high quality, however, a certain temperature dependency exists, whether because of slow drift due to aging or at high temperatures, e.g., above 130° C. Relative permeabilities that change over time or with the temperature affect the measured value and thus the measured conductivity.
Often, conductivity sensors have additional functions that are also performed based upon alternating electromagnetic fields. For example, measurements of the flow rate, the pressure, or the density should be mentioned here. These additional functions can, however, only be performed one after the other because the magnetic and electrical fields influence one another.